Diode-pumped solid-state lasers (DPSSL) with emission wavelengths in the visible range of the electromagnetic spectrum are well known laser systems. Products based on this general technology are widely used in material processing, bioanalysis, medicine, holography, spectroscopy, printing, graphics and entertainment, to name some applications.
A DPSSL with emission wavelengths in the visible generally comprises a DPSSL operating at a wavelength between 800 and 1400 nm, this emission being frequency converted (such as frequency doubled) to a wavelength between 400 and 700 nm. The frequency conversion is performed in a nonlinear optical material. For the case of continuous-wave (CW) laser sources, the nonlinear optical material is preferably located inside the resonant optical cavity of the laser source, in order to utilize for the nonlinear process the high intensity of the circulating optical field within the cavity. Intra-cavity frequency conversion, i.e. having the nonlinear optical material placed within the resonant cavity, has been found advantageous for the purpose of reaching high electrical-to-optical conversion efficiency in the overall laser system.
There are many factors motivating the construction of a DPSSL emitting visible light at a single frequency, i.e. to construct a single-frequency laser. Single-frequency means that only one longitudinal mode is allowed to oscillate in the optical resonator. One reason for making such a single-frequency laser is that many practical applications require, or would benefit from, a long temporal coherence length for the laser source. For a single-frequency laser, the coherence length may typically be 10 meters or more. Another reason is that intracavity frequency doubling of a fundamental laser signal having a plurality of longitudinal modes typically leads to the so-called green problem, or “green noise”. Green-noise is manifested by large and irregular intensity variations in the output from the laser due to gain competition between adjacent laser modes. Such intensity variations are normally highly undesired.
For the purpose of making a typical DPSSL with a standing-wave Fabry-Perot cavity single-mode (single-frequency), spectral filters may be used which introduce substantial losses for all but the desired longitudinal mode of the laser. The present invention relates to this type of single-mode laser sources, wherein the spectral filter is a Lyot-type filter.
In its simplest form, a Lyot filter consists of a birefringent (double refractive) material and a polarizing element. The birefringent material alters the polarization state of the resonating field for all longitudinal modes, and due to the spectral dispersion of the material different modes will experience different alterations of the polarization state. Typically, a linear polarization incident upon the birefringent material will lead to polarization states after passage of this material ranging from orthogonally linear to elliptical or circular, depending on the wavelength of the respective longitudinal mode. The polarizing element is then used for introducing losses for all but the desired mode. Typically, the polarizer is placed and oriented such that one linearly polarized longitudinal mode will pass virtually unaffected through the polarizer, and will thus experience very low losses, while other modes will be sufficiently suppressed in order for oscillation thereof to be avoided. In effect, only one longitudinal mode within the gain of the laser will see sufficiently low losses in order to oscillate and provide laser light output.
A general strive when designing DPSSL:s is to obtain a laser source that is insensitive to ambient temperature changes, includes a minimum number of elements, and provides stable output at the desired wavelength.
Stability problems may also be caused by spatial hole-burning in the laser gain material. As generally known in the art, the expression “spatial hole-burning” is used for the phenomenon that the gain becomes non-uniformly distributed along the propagation direction through the gain material due to depletion of the gain for the lasing mode. If any competing mode is so spectrally shifted that the standing-wave maximum thereof is separated from that of the lasing mode, it may experience a higher gain than the desired lasing mode. Mode-hops to such competing mode may be the undesired result.
The prior art has suggested some DPSSL:s incorporating Lyot-type filters for obtaining single-mode output.
U.S. Pat. No. 5,164,947 discloses a single-frequency, frequency doubled laser wherein a nonlinear material of KTP is used both for the frequency doubling and as the birefringent material in a Lyot filter arrangement. The KTP crystal is designed for Type-II phase matching where the interacting waves have orthogonal polarizations. The effective length of the KTP crystal is temperature tuned to be an integral multiple of half the fundamental wavelength. The laser also includes a Brewster plate or a polarizing reflector, which together with the KTP crystal forms a Lyot-type filter for selecting one longitudinal mode for oscillation. One embodiment exemplified is a laser generating green light at 532 nm by frequency doubling of a Nd:YAG laser having a fundamental wavelength of 1064 nm.
U.S. Pat. No. 5,381,427 discloses a single mode laser having a Lyot filter for making the emission single-mode. The birefringent crystal included in the Lyot configuration is oriented with its optical axes at an angle of 45 degrees with respect to the polarization direction produced by a polarizer. In order to obtain single-mode operation, the birefringent crystal is carefully configured such that the ordinary ray and the extraordinary ray passing through this birefringent crystal have equal losses.